Robert S. Anderson

Research

My research focuses on the processes operating at the Earth's surface and the landforms that result. Although I am interested in the entire spectrum of earth-surface processes, his most recent work focuses on the roles of rivers and glaciers in modifying large scale landscapes.

In most cases, the research projects involve both field (mapping and instrumentation) and computer modeling exercises. The present power of computing, the sophistication and miniaturization of modern field instrumentation, the increase in the availability of digital topography and of detailed paleoclimate records makes such efforts more likely to succeed now than in any time in the past. In addition, I use cosmogenic radionuclides to document the rates of landscape evolution over long time scales.

Sand transport

Anderson treats the transport of sand and dust by wind as a physics problem involving both fluid and granular mechanics. His past work in eolian transport systems entails field experiments and modeling of the evolution of mixed grainsize ripples in both cross-section and plan view, and laboratory experiments on the dynamics of sand avalanches characteristic of the lee sides of dunes.

Large-scale topography

Anderson's research on the evolution of large-scale topography was launched in the early 1990s. Work on the Santa Cruz marine terraces and mountains stimulated by the Loma Prieta earthquake is typical of what he hopes to carry out in other tectonic settings, where the topography is produced by tectonic processes, in places by repeated earthquakes, and decays by the action of a suite of geomorphic processes including weathering of bedrock, hillslope processes, and channel incision by both rivers and glaciers. Linkages between these processes lead to the interaction of tectonics and climate to produce topography at many scales, meaning that both climate history and tectonic forcing of the topography become important to constrain. The search for appropriate sites in which there exists sufficient data to constrain process types and rates has taken Anderson and his colleagues, including students, to Utah, the Sierras, Owens Valley, Panamint Valley, Kodiak Island and the Chugach Ranges in Alaska, the Karakoram in Pakistan, Nicoya Peninsula in Costa Rica and the Finisterre Range of Papua New Guinea.

Coastal terraces

A number of graduate students have been involved over the years in projects related in one or another way to the Santa Cruz coastline and adjacent mountains. The most recent work has focused on the coastal terraces, their timing (see below), and their formation in the face of both uplift of the landmass and attack by waves. Simple modeling in 1-D shows the expected evolution of a benched morphology, but this has been done with little knowledge of detailed effects of the wave forcing. In order to place more quantitative constraints on the relationship of the coastal erosion to the wave climate, former graduate student Pete Adams deployed a seismometer at the Long marine Lab in order to document the amount of energy that actually reaches the cliff. The shaking of the cliff is strongly related to the far-field wave heights and the tidal position, as one would expect. In addition, the orientation of the swell plays a role in that it determines the length of shelf across which the waves dissipate their energy prior to arrival at the coast. We hope to push this effort into the Arctic where rates of coastal erosion are many tens of meters per year.

Geochronology

In many instances the main information gap is timing in the landscape. Anderson has been involved in the development and extension of the use of cosmogenic radionuclides (10Be and 26Al in particular) in the extraction of timing in many settings. These have included dating of thin veneers of sediments on strath terraces (Fremont River, Utah; Wind River, Wyoming; Snake River, Wyoming) and marine terraces (Santa Cruz terraces, California), and documentation of bedrock lowering rates in alpine settings (Laramide Ranges, Sierras) and on bedrock river beds (Indus, Pakistan). Work led by former graduate student Lesley Perg was aimed at establishing the ages of the suite of five Santa Cruz marine terraces, which requires dealing with both inheritance and post-depositional bioturbation. The ages obtained are internally consistent, and are surprisingly young, the lowest terrace corresponding to the marine isotope stage 3, about 60 ka. In addition, the need to deal with the inheritance problem in this system led to an investigation of the littoral system. Here we have used cosmogenic radionuclides as tracers of sand within the system. Concentrations in river sands, cliff terrace cover sands, and littoral sands can be used to document the relative contributions of rivers and cliff backwearing to the littoral system, generating a long term average picture of the littoral discharge pattern in the Santa Cruz cell. Work with former graduate student Catherine Riihimaki on the Rocky Flats surface, a major remnant of pediment along the Colorado Front Range has required that we take into account multiple resurfacing events, which are documented in a depth profile of the ratio of 26Al/10Be. We find that the age of the Rocky Flats surface is diachronous, getting younger toward the mountain front where the stream responsible for incising into the surface can still resurface Rocky Flats surface.

Bedrock incision

Bedrock incision by fluvial processes, one of the least-understood geomorphic processes, has been a focus of research for some years, starting in the Fremont badlands of Utah, and moving to the Indus River in Pakistan and the Wind River in Wyoming. Anderson and his graduate students have tackled the rates of incision and the processes by which the incision is carried out using a combination of newly developed cosmogenic radionuclide dating techniques, field deployment of datalogger-based instrumentation, and modeling. Former graduate student Greg Stock used sediments and speleothems in caves along the western metamorphic edge of the Sierran batholiths as a means of documenting rates of incision over many hundreds of thousands of years. The incision history reveals very high erosion rates from 3 - 1.5 Ma, followed by much lower rates. we interpret these to reflect the passage of a knickzone past the cave sites, having been incited by tilting of the range prior to 3 Ma. The mechanism for tilting is debated, but may result from foundering of an eclogitic dense root from the range that coincides with a change int he natrue of volcanism. In addition, working with former graduate student Catherine Riihimaki and former postdoc Liz Safran, Anderson has modeled the incision of major streams issuing from the Colorado Front Range, showing that these are in a state of transient response to the exhumation of the adjacent edge of the Great Plains.

CU graduate student Mauren Berlin has launched her PhD research on the Roan Plateau of western Colorado, into which many tens of waterfalls have bitten. These were presumably cast off from the Colorado River in a pulse of late Cenozoic incision. The site is wonderful for this sort of natural experiment, as all these knickpoints exist in the same Eocene stratigraphy, and the small scale of the landscape assures that the climate is uniform. In this study we focus on not only the positions of these many knickpoints, but on the processes of waterfall retreat and subsequent cliff recession in the valleys, and the presumably very much lower rate of landscape lowering on the top of the Roan Plateau itself. In the adjacent Book Cliffs in the Cretaceous sandstones, Dylan Ward is modeling the pattern of erosion incited by incision of small streams, and is documenting the rate of cliff recession using cosmogenic radionuclides.

Glaciation

In the late 1990s, Anderson and his graduate students, in particular Kelly MacGregor and Catherine Riihimaki (with significant help from others), began to explore the evolution of higher mountain masses in the face of glaciation. Working on the small Bench glacier near Valdez, Alaska, they documented the meteorological forcing of the glacier, the glaciological response, and the sediment output as a means of constraining the erosion occurring at the bed of the glacier. The target here is understanding of the evolution of the long valley profile in the face of glaciation, which includes the generation of cirques, hanging valleys and fjords. The field work and in particular the detailed evolution of the glacier surface speed field over the summer season, has pointed toward the need to understand the glacial plumbing system and its summer evolution. We demonstrated the great utility of GPS monuments on the glacier itself in documenting the sliding hisotry over a melt season.

This project included modeling the evolution of glacial valley longitudinal profiles. Using a 1D numerical model, we showed that the flattening of glacial valleys is expected, and that the hanging of tributary valleys and the steps in trunk valley floors are straight-forward consequences of tributary systems. This modeling work has now moved into 2D glacial models, in which we simulate the evolution of glaciers on real landscapes (e.g., Yosemite, Kings Canyon, Uintas, Colorado Front Range, San Juans, Colorado [see ...instaar/rmnp page for examples]). The target here is several-fold. First, we wish to determine the climate scenario that allows a best match of the glacial footprint to the existing LGM moraine and trimline data. Second, once we have assured ourselves that the model works well, we will drive glacial histories with ELA histories, and allow the landscape to evolve in the fac e of subglacial erosion. The erosional output from the glacial will be fed to a fluvial model to allow proper coupling of glacial and fluvial systems. This effort has been augmented by analytic work on glacial erosion in which we explain the essential features of glacial long valley profiles using simplifying assumptions of quasi-steady glacial conditions and eroison proportional to ice discharge.

recent work: With CU graduate student Dylan Ward, I am attempting to determine the roles of glaciers in setting the very high relief in the Alaska range and other very high mountain masses of the world. Zack Guido is documenting the ages of moraines and adjacent river terraces in the Animas outlet glacier form the ice cap that occupied the centrak San Juans in the LGM. He is also exploring the degree to which the glacially polished bedrock of the glacial valley itself has been eroded during the last glacial cycle, using the degree to which cosmogenic radionuclides were reset.

Late Cenozoic landscape evolution of Laramide province

Another project just completed focused on the late Cenozoic evolution of the Laramide Ranges and adjoining basins in the western US. This project involved graduate student Catherine Riihimaki, and is collaborative with Liz Safran at Lewis and Clark College. Particular problems treated included the evolution of the smooth high surfaces on the shoulders and peaks of these ranges; the river and glacial profiles that incise the subsummit surfaces; and the timing of and processes involved in the exhumation of the Tertiary sediment-filled basins. The large remaining issue is the cause of the late Cenozoic exhimationof teh Great Plains and other basins adjacent to the Laramide Ranges. This remains a target of our investigation in this region.

Biological factors

Finally, former graduate student Mike Loso began tackling the thorny issue of the role of biology in modulating surface processes. He is thinking about how one might incorporate the role of rodents in transporting sediment on hillslopes, while being faithful to the elements of ecology such as where the animals prefer to live, their subterranean digging patterns, the role of predators in digging out their dens, and so on. We have now developed models of landscape evolution in the face of non-uniform occupation of the landscape by rodents.

Future Research

Anderson intends to continue work in these areas, in all cases focusing on the application of knowledge of these physical systems to an understanding of both the resultant landforms and the geological record.

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